In all honesty, I'm not quite convinced you'd need two separate networks. Just provide the security sensors, the repeaters, the gateway and your controller - that form the security system - with some sort of ups and they will keep running until the UPS has no more power left.
The none security devices will stop working when the power goes out and, unless I don't really understand your problem well. That does what I think you want to achieve? Unless you have combined a lot of sensors within a repeater it would really be my way to go. It wouldn't mean any modifications to your security hardware, because they need to be battery powered or have some sort of UPS anyway, to keep working when the power goes down.
But again it's hard to judge when you don't really understand the problem some one is trying to solve.